Katanuma is a caldera lake that was formed by a volcanic eruption 1,200 years ago. The water of the lake is very strongly acidic from a worldwide perspective. Its hydrogen ion concentration (pH) even reaches 2.4. As a result, you will not find even a single fish lurking in Katanuma. 

The surface of the lake changes to various colors depending on the weather at that time - sky blue, deep blue and emerald green. This produces a fantastic atmosphere. You must not miss Katanuma in the fall when its appearance changes. The surface of the lake, which is blue or green up to the summer, reflects the fall leaves of the surrounding mountains at this time. This reveals a lake surface of fall leaf colors. We recommend spending your time here by enjoying standup paddle boarding in the summer and hiking to see the colored leaves in the fall.

Katanuma is closed in the winter from late November to late April, so please check before you visit.